The Role of the Main Geomagnetic Field in Locating Conjugate Points

Abstract

A sample distribution of conjugate points is calculated from various models of the main geomagnetic field, evaluated at different epochs. A summary of the conclusions is: (1) For L < 4 the errors are of the order of a few tens of kilometers; (2) older models of the field such as the Finch‐Leaton give results within a few degrees of the latest more accurate models; (3) use of the eccentric dipole approximation gives errors that range from a few tens of kilometers at high latitude to several hundred kilometers near the equator; and (4) the secular change of conjugate‐point locations is very small, averaging 1 to 10 km/yr. Inclusion of Mead's boundary field for a trace from Macquarie Island to Alaska (L = S) shifts the conjugate only about 100 km. It is concluded that for L ⩾ 5 the field line passes through regions where the field intensity is weak enough to require that realistic estimates of the distortions due to plasma interactions be included in conjugate‐point determinations. (These estimates are not now available.)

An analysis of the spectra of geomagnetic variations having periods from 5 min to 4 hours

“…Langel and Svalgaard [1974] showed that local ionospheric effects are important in consideration of polar cap geomagnetic changes. Figures 9 and 10 are an attempt to show seasonal and local time effects by comparison of the spectral characteristics at two pairs of conjugately located stations [Campbell and Matsushita, 1967;Cain, 1968]. The year 1967 was used in Figure 9 because the 1965 Great Whale River data were incomplete.…”

World contours of conjugate mirror locations

“…The region was chosen to intersect: the Capetown anomaly, where a large difference in alti-REPORT 6345 rude occurs between conjugate and mirror points. A difference of about a degree is expected, owing to the difference in field models [Cain, 1968], mesh size, and methods of data contouring. Of more significance is a rather large difference in latitude, which is critically dependent on the mirror point altitude and the field line dip angle in the region between conjugate and mirror points.…”